Hi Susan,
Mickey's Not-So-Scary Halloween Party is one of my favorite special events at Walt Disney World! The party does begin at 7 PM, but according to the Disney Parks Blog, your party ticket will allow you to enter the Magic Kingdom Park as early as 4 PM. The best place to be when the party starts is anywhere in the park! Some people like to check out where characters will be greeting guests and start the party close to a favorite character. Other people like to stake out the Trick-or-Treat trails to start collecting sweets.
I love being on Main Street, U.S.A. when the Boo to You parade begins. The sight of the Headless Horseman flying down the street in front of the castle is incredible! You are correct that the parade runs twice. We usually catch the later parade when it's ever-so-slightly less crowded. I have watched the Happy HalloWishes fireworks from many different spots but I always return to the hub area in front of the castle.
We cannot exist on sweets alone, so if you are looking for a meal, I would suggest you look on the early side. The table service restaurants will not be serving during the party and several of the quick service dining locations close early.
Thanks for stopping by the Disney Parks Moms Panel. Please let us know if we can help you with any other questions!
Wishing you a magical and spooktacular vacation,
Amy H